Optical selection of split mica sheets



Dec. 4, 1923.

1,476,102 a. w. PICKARD ET AL OPTICAL SELECTION OF SPLIT MICA SHEETSFiled Aug. 25 1921 5 Sheets-Sheet l W/ ITO/M. m;

Dec. 4, 1923.

1 1,476,102 G. W. PICKARD ET AL OPTICAL SELECTION OF SPLIT MICA SHEETSFiled Aug. 23, 1921 5 Sheets-Sheet 2 Dec. 4, 1923. 1,476,102

G. W. PICKARD ET AL OPTICAL SELECTION OF SPLIT MICA SHEETS Filed Aug. 231921 5 Sheets-Sheet 5 Dec. 4 1923.

1,476,102 G. w. PICKARD ET AL OPTICAL SELECTION OF sru'r MICA SHEETS 5Sheets-Sheet 4 31mm to: fi'refafgy'mrmi at diew i OPTICAL SELECTION OFSPLIT MICA SHEETS Fi led Aug. 23 1921 5 Sheets-Sheet 5 Ill mA 770mm)Patented Dec. 4, 1923.

UNITED STATES PATENT OFFICE.

GBEENLEAI WHITTIEB PICKABD, OI NEWTON CENTER, ITASSACHUSETTS, JULIANEARTH, OF NEW YORK, N. Y., ASSIGNOBS TO WI BELES SPECIALTY APPA- BATUSCOIYANY, O1 BOSTON, MASSACHUSETTS, A CORPORATION OI NEW YORK.

OPTICAL SELECTION OF SPLIT HICA SHEETS.

Application lled August 28, 1921.

To all whom it may concern:

Be it known that we, Gnuunnmr VVHrr- 'rmn PICKARD and JULIAN BARTH,citizens of the United States residing respectively at Newton Center,Massachusetts, and New York cit New York, have invented certain Opticallection of Split Mica Sheets, the

principles of which are set forth in the fol lowing specification andaccompanying drawings, which disclose the form of the invention which wenow consider to be the best of the various forms in which the principlesof the invention may be embodied.

This invention relates to the selection of certain mica sheets or filmsfrom those which have been split to thicknesses of the order of one-halfmil to twenty or more mils from commercial mica and then cut to areas ofthe order of a fraction of a square inch to the largest sheets usedcommercially, and to the shape desired for use in the arts, as formanufacturing electrical condensers, especially of the high-potentialtype, or phonograph reproducing diaphragms. The films selected for usein manufacturing are those shown by the employment of the invention andthe execution of the method to be suitable for the desired use, theothers being discarded, at least for such use. The

selection involves princi ally considerations of the thickness of eaciifilm and its uniformity of thickness thruout its area, a commondesideratum being a collection of films all of which shall have the samethickness within limits of slight tolerated departures which may varyfor different jobs. In practice the invention is being employedprincipally in the selection of mica films as a step in the process ofmanufacturing high-tension electrical mica condensers; and itis usefulalso in all cases where mica films of definite thicknesse are desirableor even newssary, as in any structure composed of stacks of mica filmssuch, for example, as phonograph reproducers, electrical insulators,etc., etc., w ere the advantages of uniformity and thickness may beeither electrical or mechanical or both. In nmnufacturing condensers,this invention is employed preferably as the first step in Serial N0.494,002.

handling the cut mica films, since the process makes evident many faultssuch as cracks, cross crystallization of the mica, thin and thick spots,holes, etc, which only other tests such as careful eye inspection andelectrical tests will show. In fact, so thoro is the weeding out by thisinvention that such other tests may be dispensed with.

The object of this mvention is to reduce the labor cost offilm-selection, and to increase accuracy of selection of a stack offilms of uniform thickness, each of which shall have uniform thicknessand structure thruout its area; such increase of accurac involving abetter article made of suc sheets, obviating the labor cost ofrebuilding an article shown b test to be inferior on account ofinaccurate y selected mica films, and a reduction in the cost of thefilms them selves. The last is so because films are selected to aminimum thickness with a tolerance on the positive side; and since thenew method permits greater accuracy, the tolerance can be held closer,making the average thickness less. Thus two mil minimum and one-half milplus tolerance averages two and one-quarter mils, whereas two milminimum and one mil lus tolerance averages two and one-half mi a savingof ten per cent per sheet or twenty or cent. per condenser of givencapacity. saving of labor cost is also efl'ected by putting less burdenof accuracy of splitting on the splitting operative.

The invention consists of the apparatus substantially as shown in thedrawings, and in the methods of selection by the operative which areinvolved in her use of such apparatus.

0f the drawings,

Figure 1 is a lan of a mica sheet or film M to be selecte or discardedaccording to the operators observation of its thickness in the apparatuspursuant to the method hereinafter described;

Fig. 2 is a plan of the operators comparison scale involved in theapparatus and method;

Fig. 3 is an elevation of a stack of a hightension electrical condenserin winch are used the mica films selected according to the invention,and indicatin the uniformity of thickness of the mica lms (the thicknessshown for clearness being much greater than that usually employed) Fig.4 is a perspective view of the preferred form of apparatus; Fig. 5 beinga vertical section taken fore and aft of the apparatus, and Fig. 6 beinga front elevation;

Fi 7 is an enlarged horizontal section (7t of Fig. 5) of the light-box(and of its lamp-carrier). which in Figs. 4-6 is shown connected withthe lower end of the main light-chute of the ap aratus;

Fig. 8 is a detail view siiowing the combined analyzer and reflector AZand its mounting: and

Fig. 9 (modification) is a side elevation of parts of the apparatus ofFigs. 44% adapted for the use of sunlight instead of light from anartificial source as from the lamps in the lam i-carrier of Figs. 4-7.

eretofore the thiclmess-selection of mice films for use in electricalcondensers and the like has been effected by mechanical micrometering bywomen operatives. That method had the following three principaldisadvantages. First, it did not result in the selection of films all ofwhich had substantially the same thickness, because it depended uponmicrometer-manipulation which could not be uniformly accurate itreasonably high speed of operation were maintained. Secondly, it did notresult in the selection of films each of which had uniform thicknesstliruout its area, because the labor-cost involved a speed of operationwhich precluded micrometering of more than a limited number (usuallyone) of the portions of the area of a film. Thirdly, the total laborcost accompanying the old method was high, not only from the speed ofoperation limited by the use of the mechamcal micrometer, but by theliability to a bad product of condenser or condenser section whichrequired reoperation on account of the inclusion of perhaps only asingle film of unsuitable thickness. This invention makes possible agood product at low cost, obviating remaking the product. I

The method hereof has been termed optical micrometering and theapparatus an optical micrometer because they involve the measurement ofthickness of mica films with micrometer accuracy by the optical methodhereof; but these terms are not adequate because one of the greatestutilities of the invention is the selection of films each of which hasuniform thickness and structure t-hruout its area. The method will beunderstood best from a description of the ap aratus, as follows.

eferring to Fig. 4 (and Figs. 545), the apparatus comprises a generallyZ-sha-ped dark-box or light-chute Z (black-painted inside) theintermediate portion of which is vertical and mounted in the top of asuitable table or work-bench 13 before which the operative sits (left,Fig. 4). She manipulates the film to be observed on a glass table GT infront of her mounted over a holc cut thru bench B (Fig. 5), and observesat a higher level thru the peephole PH at the upper end of dark-chute Z.The principle involved is that the color of a mica fihn (or a portionthereof) when observed under certain optical conditions to be described,is a definite criterion of its thickness. \Vith the use of the apparatusshown. the color indications from mica film M thru pee hole PH are verybrilliant and sharply de ned for variations in thickness of aboutone-half mil, and shading gradually but unmistakably from onesharplydefined color to the next for the slight but important variationsof thickness of the order of a tenth of a thousandth of an inch. Thesecolors include the entire range of the spectrum and may involve mixed aswell as pure spectral colors. The colors depend somewhat on the color ofthe original source of light. \Vith daylight mazda lamps. the followingcolors are associated with the corresponding thicknesses Mlle. Color.

1 Straw-yellow. 1.5 Brown-yellow. 2. -Brown-red. 2.25 Pur le.

2.5 Dar blue.

3. Blue.

3.5 Green-blue. 4.0 Green.

4.5 Yellow-green. 5. Yellow.

Glass table GT is inset on table or bench top B over hole 0 therein(Fig. 5),in the path of the light rays upwardly traversing thedark-chute Z. The light enters chute Z from its lower end, and passesangularly and upwardly to a combined light-polarizer and reflector PZ,thence thru hole 0 in bench B, glass table GT and the mica film M placedthereon for observation. thence thru the upper part of dark-chut Z abovebench B to a combined light-analyzer and reflector AZ, and thenceangularly to the operatives eyes thru peephole PH. Glass table GT (and.mica films on it) lies in a plane transverse to the rays of lightpassing from polarizer PZ to analyzer AZ.

A light-box LB, rectangular in cross-section, is connected as acontinuation of the lower inclined part of dark-chute Z which issuppigrted by the illustrated bracket from bench and a lamp-carrier LKis mounted in the end of thi light-box LB, as shown in Fig. 7 (and Figs.4 and 5). At the upper light and ground constitutin end oi li htrbox LBwhich adjoins the lower end of rink-chute Z is mounted a plate of groundglass GG which therefore is located between the artificial source of liht on lamp-carrier LK, and polarizer P (Fig. 4). The length of light-boxLB, i. e., the distance between the artificial source of glass GG, issomewhat critical (in order to cause suitable difiused light to fall onpolarizer PZ), for the ultimate purpose of causing uniformity of lightrays thru the mica sheet M under observation on glass table GT. Theconstruction and arrangement of light-box LB, lamp-carrier LK and groundglass GG are such as to cause suitable difl'usion of light, thecriterion of which is a substantial lack of spots of greaterillumination on ground glass GG corresponding in number to the number ofartificial sources (preferably two incandescent lamps LP) in lam-carrier LK. In Fig. 7, lamp-carrier L consists of any suitable supportfor the ordinary lamp-sockets as shown for lamps LP mounted as shown,this support being provided above and below with a pair of projectingscrew heads which enter slots in four corresponding metal sprin stripsmounted on the lower end of lightox LB. In Fig. 7 lamp-carrier LK isshown thus held in place in the end of light-box LB so that lamps LP arelocated inside said box. The end inside surface of lamp-carrier LK thelower end of light-box LB, Fig. 5) is covered (Fig. 7 with nickelplatedmetallic reflecting sur aces R, with openings as shown to permit thescrewing of lamps LP in their sockets shown in Fig. 7. Also all fourside;walls of lightbox LB are lined with like nickel-plated reflectorsR. Ground glass or light-difl'user GG is mounted in the upper end (Fig.5) of light-box LB, so as to be located be tween lamps" LP and polarizerPZ. Unless the distance between lamps LP and lightdiffuser GG issuflicient, the will make two spots of light said ifl'user. (As shown,Fi 4, GGisi -located directly'transverse of t e light-b0; LB anddark-chute Z, i. e., atright anglesto the rays from the source oflight.) This distance should be such that theratios of the distancesfrom lamps LP to difierent arts of diffuser GG to one another are suntially equal to unity: and then in connection with reflectors R therewill be uniformillumination of polarizer PZ by dilfuser GG so that theillumination of the polarizer will be the substantial equivalent of thatby sunlight or of that from a greater number of sources than the twolamps LP shown. (In the apparatus in use, each lamp is a one hundredwatt lamp of the type known as daylight mazdaa With these lamps LP,there will be not only direct illumination of difl'user GG from the lambut also by reflection from all the four si es and the lower inside andof light-box LB, so that with the pro r distance between lam LP andglass G, there will be substantia 1y no spots of ligiht on the diffusingglass. The reflectors contribute to evenness of illumination on lass GG,but the distance between lamps L and glass GG is also very important IIIpreventing unevenness or spottiness of illumination. We have found thatthis distance between lamps LP and glass GG under the conditions abovedescribed, is of the order of approximately twice that of the atestdimension of the glass GG (which 1s four by eight inches in area); orthat the distance desirable is as great as is physically convenientwithout too-great reduction of intensity of light from the artificialsources LP.

The above arrangement of light-box LB, reflector R and lamp-carrier LKis entirely practicable for use with an artificial lightsource, but ofcourse may be replaced by equivalents involving such aids as complexlenses between the artificial source and ground glass GG; for example,such as condensing lenses with or without reflectors back of theartificial light-source; or, if suitable lenses alone without groundglass GG be suflicient to diffuse the light u on polarizer PZ, thenglass GG may be isensed with as an aid to difiusion, especially if suchlens arrangement produces substantially equivalent illumination ofpolarizer PZ. In th arrangement shown, however, there is total internalreflection in light-box LB on ground glass GG. While each lam LP aloneaffects glass GG only thru a sma l .solid angle, yet the reflector backof the lamps returns to diffusing-glass GG the light which otherwisewould be lost rearwardly, and the side reflectors return to diffusin-glass GG the light that otherwise woul stray oil to the sides. Thisarrangement provides complete diffusion from an artificial light-source.in addition to the described adequate intensity. The importance of theproper light-box LB 'as above is indicated by the bad effect on theappearance thru peephole PH of mica film M under observation (Fig. 4) onglass table GT, in

case the light is not adequately diflused to polarizer PZ. In such casethe parts of the surface of mica film M would be in parts of the lightfield of different intensities, the mica colors in the dark part of thefield being darker than the mica colors in a li hter part of the field,so that the film woul not appear of uniform color thruout its area evenaltho it were of uniform thickness. Such conditions also would destroyth utility of comparison scale CS (Figs. 2 and 4. to be described), incase the sample mica films on said scale were in parts of the lightfield of difierent intensity from the art of the field occupied by filmM under 0 nation.

In the modification of Fig. 9 where daylight is used instead of anartificial source as in Figs. 4-7, the diffusion of light on olarizer PZis satisfactory without any ight-box LB or diffusing-glass GG; fordaylight comes from the sky over a large solid angle, and therefore 'vessatisfacto difl'usion, as distinguishe from artifici light from a ointsource.

Light-box B hereof with its reflectors R serves in effect to multiplythe sources of artificial light and thereby to increase the intensity at66:. This box is constructed of one thirty-second inch sheet brass,nickel-plated, and has four sides, so that each of the two light-sourcesLP is reflected and re-reflected man times (from the four reflectingsurfaces B) so that in eflect there are multitudinous Iightsources. Ineneral, lam s LP themselves are locateg to cause di usion of light at GGbein located as far as possible from (1G wit out materially reducing thelight intensity thereat. The increase of effective intensity, obtainedby reflectors R, permits sulficient distance from GG to the lamps toobviate the disadvantageous light-spotting on GG.

The dark-chute Z is of one-sixteenth inch sheet iron riveted together;but instead of having interior reflectors, all its interior surfaces,from ground glas GG to top peep hole PH are lacquered to have a den lackfinish. Dark-box Z and light-box LB are secured together as shown (Figs.4 and 5) with ground glass GG at their juncture. Pieces of felt FL areinterposed as shown at the edges of the two surfaces of glass GG toprotect the same from fracturing contact with the ends of the metallicbox LB and chute Z.

Polarizer PZ is located a moderate distance (see dimensions below), asshown,Fig. 5, from glass GG to receive'light coming therethru. PolarizerPZ is located asshown at the lower end of the vertical ortion ofdark-chut Z. At the top of this vertical portion is located thelight-analyzer AZ. The polarizer and analyzer are here so called fromwhat they do to the light incident upon them. In the practical example.

shown. PZ and AZ each comprises a mirror 7 (Fig. 8) consisting of asheet of uarterinch plate glass, dead-black-lacquere on its back, i. e.,their unpainted surfaces facing one another inside dark-chute Z (Fig.5). Optical equivalents of PZ and AZ may be used, of course. aspolarizer and analyzer respectively, such as Nicoll prisms, etc., etc.PZ and AZ are located parallel to one another. Each of PZ and AZ has anle with the vertical of 32 (thirt -two egrees and thirty minutes), i.e., t e difl'erence between 90 and the polarizin and analyzing angle of57 30'. The sai vertical is the vertical position of dark-chute Z. Thepolarizing and analyzing an ice respectively are the angle between saivertical and a line perpendicular (normal) to the mirror. This normalline is midway between the incident and reflected light rays.

The area PZ and AZ and consequently the cross-sectional dimensions ofdark-chute Z and lamp-carrier LK are made relative to the areas of themica films to be tested (usually no magnification is needed), whichusually run from one inch or less on a side to films as large as threeinches by four inches, and in exceptional cases even larger. Thus, theareas of PZ and AZ, considering their proper functioning angles, aremade such that the cross-sectional dimensions of dark-chute Z andlamp-carrier LK will be suflicient to permit the part of glass table GTwhich lies within chute Z to be large enough to accommodate the largestsize of mica films to be tested, and also to accommodate the comparisonscale CS alongside the film M being tested and within' the light-path.this scale'being involved in the preferred apparatus and method. (Thedrawings hereof were made from an apparatus in use, and are in generalto actual proportion, the bench B being of ordinary work-bench height,and peephole PH being at the level of an average operatives eyes as shesits on a stool or chair in front of the apparatus.)

The actual dimensions of the up aratus in use are as follows. Polarizeran reflector PZ is seven and three-quarters inches hi h and nine andone-quarter inches wi e. Analyzer and reflector AZ is eight andthree-quarters inches high by eight inches wide. The centers of thesetwo mirrors lie on the central line of the vertical ortion of dark-chuteZ, the inside dimensions of which arez-width (Fig. 6) eight inches, andfore-and-aft thickness (Fig. 5) four inches. The upper and lowerangularly disposed ends of chute Z have the same interior dimensions.(Light-box LB also has the same interior dimensions as a continuation ofdark-chute Z3 Light-box LB and the lower angularly isposed end ofdark-chute Z are at an angle of (sixty-five degrees), Fig. 5, with thevertical (i. e., 180, less 115 which is double the polarizing angle). Inother words, the light-box is at an angle of 25 (twenty-five degrees)with the horizonta (as the level surface of bench B). The u perangularly disposed end of darkbox (extending toward the operatives eyes)is of the same angle with the vertical as the lower end, i. e., 65; or25 with the horizontal. (For the upper end, the angle is that above thehorizontal, as distinguished from the lower end which is the angle belowthe horizontal. See F ig.5.) If com arison-scale CS be not em loyed, thele t'toright width of chute Z ig. 62] may be less and it might besquare; in w ich case the dimensions of PZ and AZ would be modifiedaccordingly.

As shown in Fig. 6, polarizer PZ is mounted in dark-chute Z in such waythat it may be slid out of place (to the left) from the apparatus forthe purpose of cleaning its interior surface.

As shown in Fig. 8, analyzer AZ is mounted in a metal frame F withbent-over edges, the analyzer being slid in lace in frame F. The lowerpart of this rame F is received in a depression in light-chute Z (Fig.which acts as a ivot mountina' for it. The upper part of rame F isprovided with a slotted strip lwhich is adapted to a similarly slottedstrip 2 mounted on dark-chute Z, so that frame F ma be held to saidchute by suitable adjusts. 1e screw means. Thus analyzer AZ is readilyassembled and accessible for cleaningits 1nterior surface. and isadjustable for angle.

As shown in Fig. 4. glass table GT advantageousl may project more orless forwardly of th s hand opening HO into darkchute Z for theconvenience of the operator in placing a mica film M with her left handon the table after having removed it from a collection of previouslysplit and cut films M which may be placed on the bench or table B at theleft of chute Z. The'forwardly projecting part of glass table GT thenserves as a way on which film M may be pushed by the right hand to aposition on the rear part of the glass table within the field of thelight ra s passing up thru the glass table from po arizer PZ to analyzerAZ. As film M is pushed by the right hand into this field, the operatoris observing thru top peephole PH. and at the same time is reaching withthe left hand for the next film to be tested.

The apparatus in its simplest form may consist only of two elements,polarizer PZ and analyzer AZ in a suitable light-chute. located parallelto one another, the mica film being held between them in a plane acrossthe light-path. The relative turning movement (infra) between theapparatus and the film may be obtained either by turning the polarizerand analyzer (both or either) or by turnin the mica film; but thepreferred apparatus 18 thatshown where the ap aratus is fixed and thefilm is turned,

n that the above apparatus employs arti-- ficial light,.it is preferredover the daylight apparatus of Fig.9. But that of Fig. 9 may be employedwhen conditions oflocation are suitable, as when a north window is available, as will be described. In the case of either apparatus, the coloredimage of the mica. sheet ma be'projected upon a screen, but that usual yis not necessary on account of the great facility and certaint ofoperation b the apparatus and metho disclosed.

Whi e ma lfication or screen projection may be invo ved in an apparatusor method under the invention, yet in practice the are not usuallynecessary or desirable. hey are not usually necessar because the imageseen thru the peephole H is ample to ermit satisfactory selection ordiscar ing films of the area usually employed. They are not usuallydesirable (as a matter of the apparatus employed), because the wholearea of the comparativel lar mica film is usually desired to be wit in tie field of vision in cases where, as usual in condenser manufacture,for example, the uniformity of thickness thrnout a film is an importantdesideratum. The form of apparatus shown is that which is mostappropriate for the intended factory use, where the entire outfit may beincorporated with an ordinary workbench before which the operator sitsin an ordinary stool or chair.

In Fig. 9 the dark-chute Z is the same as before, as are also olarizerPZ, analyzer AZ and glass table T. Here, however, the lower inclined endof chute Z receives the light rays of naturall -difiused da lightentering as from a nort 1 window as conveniently via mirror I mounted ina horizontal position by means of any suitable sup port and having a.silvered ack. In this case, silvered mirror I replaces lamp-carrier LKof the preceding figures, and no li htbox LB or ground-glasslight-diffuser G is needed, on account of the intensit and diffusion ofthe daylight employ As before, the operator observes (thru top peepholePH) the image in analyzer AZ of a mica film which has been laid on lasstable GT within chute Z alongside, pre erably, of a comparison scale CSto be described.

The general design of the preferred aparatus is such that onl such lightrays roin the source of uni orm illumination, ground lass GG, reach theeye at PH, as retain, su tantially, their parallelism while undergoingthe various optical changes. It is accomplished b having as small acrosssectional area of 'ght-chute as will accommodate the largest filmto be inspected, and by having as great a distance as is convenientlyand po arizer PZ, between polarizer PZ and analyzer AZ and betweenanalyzer AZ and the eye at P The optical phenomena involved demand thatfor most intense colors, the analyzer AZ be fixed in relation to thepolarlzer PZ so that when nothin intervenes between olariz'er PZ and anayzer AZ, there mt er a maximum or a minimum transmission (orreflection)of polarized light by the analymr AZ. The position of the anal zer AZ inthe preferred apparatus is thato maximum ret ible between the groundglass GG p flection, i. 8., in the arrangement shown. If

that part of the apparatus above the table B were swung thru ahorizontal angle of 90, we would have a case of minimum reflection. Thetwo different arrangements give, for graded thicknesses of mica, twodifferent sets of colors which are substantially complementary sets ofcolors. In a position of analyzer AZ halfway between the abovestatedpositions of maximum and minimum reflection respectively, colorlessnessof mica results when a film is inspected between analyzer and polarizer,and such colorlessness grades into one set or the other of colors as theanalyzer is turned respectively towards the stated positions of maximumor minimum reflection. In the preferred apparatus, the maximumreflection position is used because the set of colors corres onding to arange of films between one an four mils in thickness is a more sharplydefined and more easily recognizable set of colors than thecomplementary set which would be observed at the position of minimumreflection.

The preferred method of selection, using any suitable form of apparatusof the invention, involves the use of comparison scale CS or itsequivalent, shown separately in Fig. 2 and in assembl in Fig. 4 on glasstable GT in the line of t e light rays passing up thru chute Z frompolarizer PZ to analyzer AZ. Thisscale consists (Fig. 2) of a support ormounting strip CSwhich may be of cardboard and has light-holes E thereinto re ceive light from glass table GT (Fig. 4) on its way upward toanalyzer AZ. One or more (preferably several) mica filmsM (Fig. 1) ofknown thickness-condition are ummed to this cardboard stri over holes sothat light rays will pass t ru the films on their way u thrubench-opening 0 (Fig. 4) and glass ET to analyzer AZ. Such a comarison-scale may be made up and installe for each different job, as forexample, for the manufacture of a given electrical condenser whichrequires mica films having no more nor less than a given limited rangeof thicknesses. In the comparison scale shown (Fig. 2), thisthickness-tolerance is between one mil and two mils as indicated by themilmarking for" the o erators guidance on the strip CS. The to lowing isthe manner of making and installing a comparison scale US for a given'ob. First. the strip US with light-holes is placed and fixed (as bygluin to the desired portion of glass table G as at the left as shown inFig. 4, and oriented rectilinearl with the glass table. Then a mica filmknown to be of uniform thickness thruout its area and known to be of athickness corresponding with the notation on the left of a given hole Ein strip CS (Fig.2), is placed over the hole E opposite such notation.(In Fig. 2, all but one of the mica films are shown in place over theholes, and one hole is left uncovered at the bottom 0 )posite thenotation 1.0, meaning one mil.) Assume that the film in question is ofknown one-mil uniform thickness, and has been laced temporarily but notsecured over hole l opposite the note 1.0. The initial angular positionflat on glass table GT in which such film may be placed temporarily mayor may not be the position which will result in showing color to theinstallers eye as he looks in the top peephole PH; or if color doeshappen to show in such initial angular position, that position of thefilm may or may not be the position which will bring out the color mostintensely. In either case. the installer then turns the film (flat onCS) in order to determine whether the intensity of the color may beincreased. If the turning results in a decrease of intensity, then thefilm is turned back to the position resulting in greatest intensity. Inany case, the film is gummed to strip US at the angular position flat onit at which it shows the greatest intensity when viewed thru peepholePH. The turning of the film may be a maximum of 45 (fortyfive degrees),although it usually is less; but the turning is done in all cases evenif only slightly. because the color-appearance is most intense only whenthe mica film, flat on glass table (iT, occupies a definite angularposition in its plane transverse to the light rays passing thrudark-chute Z. This is due to the fact that the relative position of theaxis of crystallization to the polarized rays determines the change inthe polarized rays.

With the complete installation of comparison scale US as above by theinstaller, the apparatus is ready for the execution of the method by theoperative which involves a comparison of the appearance of a given filmM (Fig. 4) of unknown thickness to be measured, with the sample films ofknown thickness-conditions on scale US, by inspection thru peephole PH.

The colors (at maximum intensity) of different films vary widely withinvery narrow limits of film-thickness, so that great accuracy ofselection may be obtained, i. e., as great as within fractions ofthousandths of an inch, i. e., of the order of ten-thousandths of aninch.

First, the operative. with the fin ers of her left hand. picks up a filmfrom the mass of previously split and cut films M to be meas ured whichlies on bench B at the left of chute Z (Fig. 4). She places this film onthe forwardly-projecting portion of glass table GT, removes her lefthand (which goes back to the left for the next film) and with theforeand middle fingers of the right hand she pushes the film (to beinspected) into position on glass table GT alongside of scale CS. Thenwith the same two righthand fingers (while the left hand is pick- HitIll]

, ing up a second film from the left and while she is looking thrupeephole PH) she turns the first film until she observes thru pee holePH the most intense color from this first film, precisely as theinstaller did in placing the sample films on the comparison scale. Shethen instantly com ares this color (yet looking thru peephole with thoseof the sample films on scale S, observes whether it matches the color ofone of the films on the scale, i. e., whether it. is within thetolerance limits of the scale (or, if desired, observes the notation ofthickness alongside the sample film which most nearly approximates the aparent color of the film under test), and t ereupon with the sameright-hand fingers she pulls the tested film from table GT and drops itinto a good or bad box, as the case may be (or places it in acompartment of a gradin -box RX for tested films located on bench at theright of chute Z, which compartment is milmarked to correspond with thematched sample of scale CS). Discard is made not only of too-thick ortoo-thin films, but also of each film which shows a difference of colorsat dilferent parts of its area, for that means lack of uniform thicknessof such film. These non-uniform films, which may be wholly undesirablefor use in condensers, preferabl are dropped into a different receptaclerom those films which are rejected for being too thick or too thin, asthe latter may be useful in other condenser-jobs. In Fig. 3 is shown aportion of a condenser stack consisting of alternate elements ofselected mica films M and dielectric or insulating sheets 1) as oftinfoil. It is such mica films which it is of the greatest importance tohave not only of like thickness one to another, but each one of uniformthickness thruout its area in order to act equally at all points withrespect to the electrical stresses involved in the operation of-thecondenser. This method of test shows up minute mica cross-crystals (ifthey exist) embedded in or between the laminae of the film (the filmsare not usually split to a single laminae but 0:11 to a thickness of oneor more mils) and such a condition of course is a source of electricalweakness such that if such film were incorporated in a condenser, itmight cause breakdown of the stack. This method also shows up holes,which are of course zero thickness of mica. All such films, of course,may be drop into the box of absolute discards, as in t e case of thoseof non-uniform thickness thruout their area.

This method does not show up vertical cracks in the films (which do notinvolve any variation in thckness of film) if the crack is clean, i. e.,has no frayed edges, to any better advantage than direct visualinspection; but this is not alpractical limitation because cracks usua yhave frayed edges which means non-uniform thickness. The method verybeautifully shows up such dangerous variations in thickness of a givenfilm as those which frequently are caused by the initial splitting orcutting of the sheets, i. e., the breaking away of one or more of thelaminae for a part of the area of the film, usually adjoining its edge,this being the case of the most frequent and most seriou departure fromthe desired uniformit of thickness thruout the film-area, and beingunobservableby any other practicable method so far as we know..

Inasmuch as the last-named defects usually occur at the edges of thefilms, the cross-sectional area of light-chute Z is made greater thanthe total area of the film to be observed, in order to permit inspectionthru pee hole PH of its area at its edges.

rom the above, it appears that the method consists in obtaining micafilms each of substantially uniform thickness thruout its area and allof substantially the same thickness within the tolerance limits for agiven job for use in condensers and other mica articles, by selectingsuch films as are shown, by turnin the film into its critical positionunder t e optical conditions employed, to be of. said thickness and ingrouping the films which successf ull pass the test. Such grouping maybe eit ier by placing the good films in the compartments of agrading-box RX (Fig. 4), graded in accordance with comparison scale CS,or by grouping all the good films in one box for use in a given jobwhich involves the tolerance limits corresponding to the comparisonscale CS. Also such grouping may be of the films outside of thetolerance limits of a given job", but good as to uniformity of thicknessthruout area, for other jobs; or of the too-thick films for furthersplitting and subsequent retesting hereby.

While the scale CS mounted within the field of light in chute Z ispreferred, yet a comparison scale may be used which has the 'ven colorspainted upon it and which may mounted within the range of vision of theoperator, as on the outside of chute Z above hand opening H, i. e., outof the field of light in chute Z. But such a painted scale so locatedmanifestly lacks the advanta of the illustrated scale CS. t may bepossible for an operative to become so skilled as to make unnecessa theuse of any scale, placing reliance sole y on color-memor but this is notpreferred on account of t e obvious liability to error and the vitalimportance of preventing such error, for even a single too-thin film 1fem bodied in a condenser stack might result in a breakdown of acompleted condenser which might require very large labor cost inrebuilding the stack.

III

In the use of the mica-measuring apparatu hereof, there is a phenomenonwhich may be termed color-octaves; that is, for example,-a very thinfilm may indicate a color somewhat s'milar to that of a very thick film,but the similar colors of different octaves are so different in qualityor rich trees that mistakes cannot be made. i In addition, they lie sofar apart in thickness of mica that the touch sense of stiffness easilydistinguishes them.

The invention is based on the facts long known to ph sicists that micaand other films would siiow colors under certain optical conditions ofpolarized light and that such colors have a direct relation to thethickness of the film under observation; therefore we make no claim tosuch d'scovery. Our invention is directed to mica films which previouslyhave been 5 lit to comparatively slight thicknesses suita le for use nthe arts, and which also preferably have been cut (as by suitable die'ngmachinery) into shapes suitable for particular uses in the arts as inelectrical condensers (the dimensions of the apparatus hereof beingadapted in a given case to the area of the split and referably cutfilms) and the apparatus an method are concerned in the selection offilms of des'rahle thickness and each of uniform thickness thruout itsarea from quantities of such previously split (and preferably cut)films, and in the rejection of films which lie outside (too thin or toothick) of the thicknesses to be tolerated for a given condenser or otherjob, and (in condenser work) in the rejection also of each film ofnon-uniform thickness thruout its area; and all this involves thedetermination of the thickness of the films and the uniformity ofthickness of each of them from the apparent colors thereof when observedunder the optical conditions described.

Ordinary skilled splitting is so accurate that most of the films which.come to the apparatus of this invention are within the usual tolerancelimits for the average condenser, and the relatively few which are faroutside (havin accidentally 0t by the splitting operation? are readilydiscarded by the sense of feeling; so that, aside from the discardingofvfilms which are just out of range of thickness, the principal utilityof the invention (involving the cost reductions above stated) is in thediscard of films which are non-uniform thruout their area.

We claim 1. In the art of manufacturing articles comprising split micafilms, the apparatus for selecting films having a condition as tothickness which is suitable for such articles, from a quantity of such.films having um known thickness-conditions, which com rises alight-chute having dark interior wa s and a cross-sectional area largerthan the area of the films to be handled, said chute having a centralportion mounted to be generally vertical and having upper and lowerinclined end portions, the whole being generally 2- shaped, and the endof the upper inclined portion being open to transmit light to the eyesof an observer; at light-box mounted with and constituting an extensionof the lower inclined chute-portion and having reflecting interiorsurfaces; a lamp-carrier adapted to receive a lamp near the end of thelight-box remote from the end of the lower inclined chute-portion; alight-polarizer constructed and mounted to receive light from saidlight-box and transmit it upwardly thru the central portion of thelight-chute; a light-diffuser mounted between the lamp and polarizcr andsufiiciently remote from the lamp to prevent substantial light spots onsaid diffuser; a light-analyzer constructed and mounted to receivelightfrom the polarizer and transmit it thru the upper inclinedchute-port ion to the observer's eyes; said light-chute having anopening in the side of its central portion between the polarizcr andanalyzer and facing in the same direction as the upper inclinedchute-portion; and a transparent film-support mounted in and transverseof said light-chute below said opening.

2. Apparatus for selecting mica films of desired thickness-condition,which comprises a lightchute having a central portion and two endportions inclined thereto, the whole being generally Z-shaped and havingdark interior surfaces; one end of the chute being open to transmitlight to the eyes of an observer; a light-box mounted with andconstituting an extension of the other inclined end of the light-chute,said light-box having reflecting interior surfaces; a source ofartificial light mounted to supply light to said light-box; alight-polarizer construe and mounted to receive light from the lightboxand transmit it thru the central portion of the light-chute; a,lightranalyzer constructed and mounted parallel to said polar izer toreceive light therefrom and transmit it to the observers eyes throu hsaid open end of the chute; the angles 0? the inclined end portions ofthe light-chute conforming generally with the angles of the incident andreflected rays of the parallel larizer and analyzer; said light-chutebeing provided with ahandopening between the polarizer and analyzer andpermitting manual turnmgof the mica films inside the light-chute durinobservation through the open incline end portion of the Z-shaped chute;and a. light-diffuser mounted in the path of light between the lightsource and the polarizer.

3 Apparatus for selecting mica films of desired thickness-condition,which coim prises a work-table or bench and a subtantially Z-shapedlight-chute combined therewith; the central portion of said chute beingsup rted generally vertically upon and exten ing thru, above and belowthe top of the said work-table; the up r inclined end of said chutebeing directeg forwardly from the work-table and open to the eyes of anobserver; the lower inclined end of said chute being directed in theopposite direction and open to asource of llght; a transparentmica-film-support mounted on the table transversely of the light-chute,the

' latter being provided with a hand-opening above said support facingforwardly a lightpolarizer constructed and mounte to receive theentering light and transmit it upwardly thru the vertical centralportion of the chute, the transparent su port and the mica film thereon;and a 1 ht-anal zer constructed and mounted to receive the light fromsaid polaraizer and su port, and transmit it thru the upper inc inedportion of the light-chute to the eyes of the observer.

4. pparatus for selecting mica films of desired thickness-condition,which comprises a light-chute having a central portion and twoend-portions inclined thereto, the whole being generally Z-shaped; oneend of the chute being open to receive 1i ht from a suitable source, andthe other ent l being open to transmit light to the eyes of an observer;a light-polarizer constructed and mounted to receive the entering lightand transmit it thru the central portion of the chute; a li ht analyzerconstructed and mounted para el to said polarizer to receive the lighttherefrom and transmit it thru the observers chute-end; the angles ofthe inclined end portions of the lightchute conformin gnerall with theangles of the inci cut and re ected rays of the parallel polarizer andanalyzer; and a transparent mica-film-sup ort mounted in a planetransverse to the light-rays assing from the polarizer to the analyzer;t e central portion of the chute being provided with an opening in oneside above said suport and constructed to permit manual turn- 111g ofthe mica films inside the light-chute during observation through theopen inclined end portion of the Z-shaped chute;

5. Apparatus for selecting mica films of desired thickness-conditions,from a quantity of such films having unknown thicknessconditions whichcomprises a light-chute having a cross-sectional area reater than thearea of the films to be hand ed and having a central portion supportedenerally vertically, and two inclined en portions, the whole beinggenerally Z-shaped; a lightolarizer constructed and mounted to receivelight thru the lower inclined end of the chute and to transmit it upthru the vertical portion of the chute; and a light-analyzer conrays andtransmit them thru structed and mounted arallel to said polarizer toreceive light t erefrom and transmit it to the eyes of an observer thruthe u per inclined portion of the light-chute, t e end of which isprovided with an opening which permits passage of such light; thecentral generally vertical portion of the light-chute being providedwith a handopening located between the polarizer and analyzer andconstructed to permit manual turnm of the mica films inside thelightchute uring observationthrough the open inclined end portion of theZ-shaped chute.

6. In the art of manufacturing apparatus comprising split mica films,the apparatus for selectin films havin a condition as to thickness w ichis suitab e for such articles, from a quantity of such films havingunknown conditions as to thickness, which comprises a light-chute havina cross-sectional area greater than the fi s to be selected; alight-polarizer constructed and mounted to receive light ra s andtransmit them thru said chute; a li "ta-analyzer constructed and mountedpara lel to said polarizer to receive said rays and transmit them to theeyes of an observer, one chute-end being provided with an opening topermit li ht and the other end being open to permit 0 servation; saidchute being provided with a hand-opening between the polarizer and analzer and constructed to permit manual tumm of the mica films inside thelightchute uring observation of rays from the analyzer; a transpareptsupport mounted between the polarizer and analyzer in a location accessile thrqugh said hand-opening; and a mica film of known-thicknes conition mounted on said support for comparison with a mica film underobservation on said an port.

In the art of manufacturing articles comprising split mica films, theapparatus for selectin films havin a condition as to thickness w ich issuita le for'such films from a quantity of such articles having unknownconditions asto thickness, which comprises a light-chute having across-sectional area which is larger than the area of each flhn to beobserved; a light-polarizer constructed and mounted to receive light thechute; and a li ht-analyzer constructed and mounted para el to saidpolarizer to receive said rays and transmit them to the eyes of anobserver; said light-chute being provided with a hand-opening betweenthe polarizer and analyzer and constructed to permit manual turnin ofthe mica films inside the 11 htchute uring observation of the rays mmthe analyzer.

8. In the art of manufacturing articles comprising split mica films, theapparatus for selectin films having th1ckness-cond1- tions suitab e forsuch articles, from a quantity of such films having unknown thicknessconditions, which comprises a li ht-chute; a light-polarizer constructedan mounted to receive light and transmit it thru the chute; alight-analyzer constructed and mounted parallel to said olarizer toreceive thelight thru the chute rom said polarizer and transmit it tothe eyes of an observer; said light-chute being provided with ahandopeningbetween the polarizer and anal zer constructed to permitmanual turning 0 the mica films inside the light-chute duringobservation of the rays from the analyzer.

9. In the art of manufacturing articles comprising split mica films, themethod of selecting films having a condition as to thickness which issuitable for such articles from a quantity of such split films havingunknown conditions as to thickness, which consists in placing the latterfilms in a plane transverse to light rays passing from a polarizer to ananalyzer, alongside a comparison film of known thickness-condition, bothfilms being oriented in their planes so that they appear to be highlycolored to the eye of an observer receiving light from the analyzerwhich has passed thru said films; then comparing the color of the filmunder test with that of the comparison film; and selecting for thearticle of manufacture those of the films under test which are shown bysaid com arison to be of the desired thickness-con ition.

10. In the art of manufacturing articles comprising split mica films,the method of selectin films having a condition as to thickness which issuitable for such articles, from a quantity of such split films havingunknown conditions as to thickness, which consists in placing the latterfilms in a plane transverse to l ght rays passing from a p0 larizer toan analyzer; causin relative rotation of the optical system am? each ofsuch films until the latter is insuch angular osi tion in such planethat it appears to ave the most intense coloring to the eye receivmgrays from the analyzer which have passed thru the films, comparing suchcolor with a color known to corres ond with that of a mica film of givencon ition of thickness; and selecting for the article of manufacturethose of the films observed which are shown by said comparison to be ofthe desired condition of thickness.

11. In the art of manufacturing articles comprisingmsplit mica films,the method of selecting fi s having a'condition as to thickness which issuitable for such articles, from a quantity of such split films havingunknown conditions as to thickness, which consists in placing the latterfilms in a plane transverseto light rays passing from a larizer to ananal zer; turning said fil iiis in such plane unt' they assume suchangular positionwas to appear to have the most intense coloring to theeye of an observer which is receiving the rays from the analyzer whichhave passed thru the films; and selecting for the article of manufacturethose of the films observed which are shown by such apparent coloring tobe of the desired thickness condition.

12. In the art of manufacturing articles comprising split mica films,the method of selecting films having a condition as to thickness whichis suitable for such articles, from a quantity of such split films haviunknown conditions as to thickness, whic consists in placing the latterfilms in a piano transverse to light rays passing from a polarizer to ananalyzer; causin relative rotation of the optical system an each of suchfilms until the latter is in such angular position in such plane thatthe films appear to be of the most intense coloring to the eye of anobserver which is receiving the rays from the anal zer which have passedthru the films; am? selecting for the article of manufacture those ofthe films observed which are shown by their apparent coloring to be ofthe desired condition of thickness.

13. In the art of manufacturing articles comprising split mica films,the method of selecting films having suitable uniformity of thicknessthruout their area from a quantity of such split films, which consistsin placing the latter films in a plane transverse to light rays passingfrom a polarizer to an analyzer; causing relative rotation of theoptical system and each of such films until the latter is in suchangular sition in said plane that they appear to of the most intensecoloring to the eye of an observer receiving light from the anal zerwhich has passed thru said films; an selecting for the article ofmanufacture those of the films observed which show the uniformity ofcolor thruout their area which corresponds with the desired uniformityof thiclmem.

14. In the art of manufacturi electrical condensers comprising split ancut mica films, the method of selecting films having a suitableuniformity of thickness thruout their area, from a quantity 'of suchsplit and cut films, which consists in plac' the latter films in a planetransverse to ht rays passing from a polarizer thru the entire film-areato an anal zer; turning said films in said plane until 1: ey a pear tohave the most intense coloring to t e eye of an observer which isreceiving the rays from the analyzer which have passed thru said films;and selecting for the condenser those of the films observed which showthe uniformity of color thruout their area which corresponds with thedesired uniformity of thickness. s

15. In the art of manufacturing articles comprising split mica films,the method of sel mg films having a condition as be thick ill ness whichis suitable for such articles, from causin relative turning of the filmsand the a quantity of such films having unknown optica system.conditions as to thickness, which consists in In testimony whereof wehave signed our 10 causing light ravs to pass from a light-ponames tothis specification.

I larizer thru the faces of such films to a lightanalyzer While visuallyobserving the light GREENLEAF WHIT'IIER PICKARI). rays from the analyzerand simultaneously JULIAN EARTH.

Certificate of Correction.

It is hereby certified that in Letters Patent No. 1,476,102 grunt/edDecember 4, 1923, upon the application of Greenleaf Whittier Iirkar ofNewton Center, Massachusetts, and Julian Barth, of New York, N. Y., foran improvement in Optical Selection of Split Mica Sheets, errors appearin the printed specification reguiring correction as follows: Page 9,line 110, claim 7, for the word films the said Letters Patent should beread with these corrections therein that the same ma conform to therecord of the case in the Patent Ofiice.

Signe and sealed this 5th day of February, A. D., 1924.

[m] WM. A. KINNAN,

' lr-h u f'nmntz'ssio'rwr 0f Patents.

read articles, and line 111, for the word articles read films; and 1815;

